As with many homebrew prototypers, who use photoresist, I have always had a problem getting totally opaque patterns on transparencies from my inkjet printer. I am, however, unique in that I invert black/white, and then print on litho film to guarantee highest contrast,which reduces the amount of ink that the printer has to lay down. That minimizes the large areas of inking that suffer the most from lack of opacity.
All the while, I have used Windows 7 or XP to control my Canon ip6700D and set all colors to maximum, for maximum inking. On Windows, the max. ink density is +50%. On a hunch, I brought over my pdfs to my Linux pc, on which I use Turboprint to control the printers. Using Turboprint Control Panel, I can set the maximum inking to +400%! I did this on the same board pattern I ran off Windows. Wow! The ink was piled so high, that when it dried, it had spider patterns. The pattern, however, maintained precision. The pattern was 100% opaque! I reduced maximum inking to 300%, and that was still to much ink.
I can't speak to printers other than the Canon ip6700D. If you've got one, or maybe a similar model, and access to a Linux pc with Turboprint printer control, you won't have any more trouble getting 100% opacity of your pcb patterns. I have no monetary interest in Canon or Turboprint products.
I'm also willing to be educated by anyone with an ip6700D on WIndows who can show us how to get the 400% max. ink deposit without resorting to Linux.
YMMV,
Stan